Methods and systems for performing superheated reactions in microscale fluidic systems

ABSTRACT

The present invention is generally directed to methods and systems for performing chemical and biochemical reactions at superheated temperatures by carrying out the reactions in microscale fluidic channels. Also provided are applications of these methods and systems, as well as ancillary systems for use with these methods and systems in monitoring and controlling the performance of the methods of the invention.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of U.S. Ser. No. 09/023,693,filed Feb. 13, 1998, which is incorporated herein by reference for allpurposes.

BACKGROUND OF THE INVENTION

[0002] Microfluidic analytical systems have been gaining substantialinterest for use in performing myriad chemical and biochemical analysesand syntheses. For example, such systems have been described for use inperforming nucleic acid amplification reactions (See U.S. Pat. Nos.5,498,392 and 5,587,128), for use in performing high throughputscreening assays, e.g., in drug discovery operations (See commonly ownedPublished International Application No. WO 98/00231), for use in nucleicacid separations (See Published PCT Application No. WO 96/04547), andfor a variety of other uses. These microfluidic systems generallycombine the advantages of low volume/high throughput assay systems, withthe reproducibility and ease of use of highly automated systems.

[0003] Because of the above advantages, it would generally be desirableto expand the applications for which these systems are used, as well asexpand the scope of the advantages which such systems offer overconventional assay systems, e.g., faster throughput, lower volumes, etc.One area of particular interest is the performance of temperatureresponsive reactions, e.g., reactions that progress faster at highertemperatures, or require a substantially elevated base temperature tooccur. In many cases, desirable chemical and biochemical reactions canbe substantially expedited by performing the reaction at substantiallyelevated temperatures. However, in fluid systems, and especially aqueousfluid systems, a practical limit on the temperature of the operationgenerally is imposed by the boiling point of the fluid. For example, inaqueous systems, the boiling temperature of the fluid at or near 100° C.is the effective maximum achievable temperature at ambient pressures ofapproximately 1 atm.

[0004] In order to perform reactions that utilize or even requiretemperatures that are above the boiling point for the fluid reactants,the use of pressure sealed reaction vessels are typically required toelevate the boiling temperature of the fluid by increasing the ambientpressure for the reaction. Unfortunately, in many reaction systems, theuse of such sealed containers is impracticable. For example, inmicrofluidic systems, the extremely small scale of the fluid carryingelements of the system and thus the fluid volumes used, as well as thenature of the fluid transport systems employed, typically prohibit theuse of pressure sealed reaction containers.

[0005] Additional concerns are raised in microfluidic systems where thepresence of a bubble or bubbles, e.g., from inadvertent boiling offluids within the system, can have extremely detrimental effects on thesystem by significantly fouling or plugging channels of the system. Suchfouling can inhibit or completely block the ability to move fluidsthrough the channels of the system, as well as the ability to monitorthe contents of the system, e.g., using amperometric or potentiometricmeans. Further, in microfluidic devices employing electrokineticmaterial transport systems to move materials through the microscalechannels of the device, such fouling can result in a cascade effectwhere the blockage results in higher current densities through theremaining portions of the channel which leads to greater heating. Thisgreater heating, in turn, leads to more bubbles within the channels fromboiling of the fluids.

[0006] It would therefore be desirable to be able to perform reactionsat temperature levels that are at or substantially above the boilingpoint of the fluids used in the reaction, while benefiting from theadvantages of microfluidic systems. The present invention meets theseand a variety of other needs.

SUMMARY OF THE INVENTION

[0007] The present invention is generally directed to methods andsystems for performing chemical and biochemical reactions at superheatedtemperatures by carrying out the reactions in microscale fluidicchannels. Also provided are applications of these methods and systems,as well as ancillary systems for use with these methods and systems inmonitoring and controlling the performance of the methods of theinvention.

[0008] In one aspect, the present invention provides methods forperforming reactions at superheated temperatures, which comprise placingat least a first reactant in a microscale fluidic channel. An effectivelevel of energy then is applied to the fluid in the microscale channel,whereby the fluid is heated to a superheated temperature without boilingthe fluid within the channel.

[0009] In a related aspect, the invention also provides a method forperforming a reaction at a superheated temperature, which comprisesproviding a substrate having at least a first microscale channeldisposed therein. The substrate is in communication with an energysource that delivers the sufficient level of energy to the contents ofthe microscale channel to heat said contents to superheatedtemperatures. The first reactant then is placed into the microscalechannel, and the sufficient level of energy from said energy source isapplied to the microscale channel to heat the contents of the channel tosuperheated temperatures.

[0010] In a further aspect, the present invention also provides systemsfor carrying out the methods described herein. In particular, thesesystems comprise a microfluidic device that includes at least a firstsubstrate having a microscale channel disposed therein, where themicroscale channel has at least first and second unintersected termini.A heating system is also included to apply energy to the microscalechannel to heat a fluid in the channel to superheated temperatures,without boiling the fluid in the channel. Further, a controller is alsoprovided for maintaining the energy applied from the heating system tothe microscale channel at a level sufficient to superheat contents ofthe microscale channel without boiling the contents of the channel.

BRIEF DESCRIPTION OF THE FIGURES

[0011]FIG. 1 illustrates an expanded view of a microfluidic device andchannel structure for performing superheated reactions according to thepresent invention.

[0012]FIG. 2 is a temperature profile for fluids disposed withinmicroscale channels of a microfluidic device while the device wasglobally heated in an oven.

[0013]FIG. 3 is a profile of the temperature of fluid, as calculatedfrom the fluid conductivity, in a microscale channel versus time whileincreasing current was incrementally applied through the channel.

DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention generally provides methods and systems forperforming chemical, biological or biochemical reactions in fluidsystems. More particularly, the methods and systems of the inventionpermit the performance of such reactions at superheated temperatures,i.e., above the boiling temperature for the fluid at the ambientpressure of the system. Such methods and systems are generally useful inspeeding reactions that are temperature dependant, as well as forcarrying out reactions that require temperatures in excess of theboiling temperatures for the fluids used. Examples of such reactionsinclude reactions that require a thermal denaturation step such asenzyme inactivation reactions, nucleic acid amplification reactions, andthe like.

[0015] Typically, the methods and systems of the invention operate byplacing the fluid reactant or reactants into a microscale fluidicchannel, and heating the fluid within the channel to superheatedtemperatures. Included among the benefits of the present invention isthe fact that the systems and methods described herein allow fluidcontained within one or more microscale channels to be heated tosuperheated temperatures without boiling the fluid that is containedwithin those channels. In addition to the benefits normally available inperforming superheated reactions, e.g., higher reaction temperatures inaqueous systems, the present invention is also particularly useful inmicroscale systems where the generation of a bubble within a channel canhave fatal consequences for the system, e.g., blocking materialtransport, current flow, etc. through that channel.

[0016] Without being bound to a particular theory of operation, it isbelieved that the microscale channels fabricated by traditionalmicrofabrication methods, e.g., photolithography, chemical vapordeposition, wet chemical etching, injection molding, etc., have surfacesthat are resistant to bubble nucleation during the boiling process. Assuch, fluids within channels having such surfaces will not boil at theirexpected boiling points.

[0017] As used herein and as noted above, the term “superheatedtemperature” for a given fluid or mixture of fluids, refers to atemperature that is greater than the temperature at which the particularfluid or fluids will boil at the ambient pressure for the system. Inpreferred aspects, the present invention provides heating of fluids totemperatures more than 5° C. above the boiling point of the fluids, morepreferably, greater than 10° C., 20° C., 30°, 40° C. and even 50° C.over the boiling point of the fluids. For example, in the case of a purewater system, superheated temperatures are generally greater than 100°C. at 1 atm pressure. In many instances therefore, the methods andsystems of the present invention provide heating of fluids, andparticularly aqueous fluids, within microscale channels to temperatureswell in excess of 100° C., 105° C., 110° C., 120° C., 130° C., often inexcess of 140° C., and in some cases in excess of 150° C., withoutboiling the aqueous fluid that is contained within the heated channel.Of course, variations in ambient pressure also bring correspondingchanges in the boiling temperature of fluids at that pressure. As usedherein, the term “aqueous system” generally refers to a fluidcomposition that is made up substantially of water, e.g., greater than30% (v/v), typically greater than 50%, often greater than 80%,preferably greater than 90% and more preferably greater than 95% water(v/v). Although described in terms of aqueous systems, it will beappreciated that the present invention is equally applicable tonon-aqueous systems, e.g., organic solutions, etc.

[0018] As described above, the methods and systems of the presentinvention operate through the placement of the fluid reactants into amicroscale channel that is typically incorporated into the body of amicrofluidic device. As used herein, the term microscale or microfluidicrefers to a structural element, and typically a fluidic element, e.g., achannel or chamber, which has at least one cross-sectional dimension,e.g., depth width or both, that is between about 0.1 μm and about 500μm, preferably between about 1 μm and about 200 μm, and in many cases,between about 10 μm and about 100 μm.

[0019] The body structures including the microscale channel or channels,as described herein, can be fabricated from a variety of differentsubstrate materials. For example, in many instances, the body structureand channel or channel networks are microfabricated. As such, substratematerials are often selected based upon their compatibility with knownmicrofabrication techniques, e.g., photolithography, wet chemicaletching, laser ablation, air abrasion techniques, injection molding,embossing, LIGA, and other techniques. The substrate materials are alsogenerally selected for their compatibility with the full range ofconditions to which the microfluidic devices may be exposed, includingextremes of pH, temperature, salt concentration, and application ofelectric fields. Accordingly, in some preferred aspects, the substratematerial may include materials normally associated with thesemiconductor industry in which such microfabrication techniques areregularly employed, including, e.g., silica based substrates, such asglass, quartz, silicon or polysilicon, as well as other substratematerials, such as gallium arsenide and the like.

[0020] In the case of semiconductive materials, it will often bedesirable to provide an insulating coating or layer, e.g., siliconoxide, over the substrate material, and particularly in thoseapplications where electric fields are to be applied to the device orits contents.

[0021] In alternate preferred aspects, the substrate materials willcomprise polymeric materials, e.g., plastics, such aspolymethylmethacrylate (PMMA), polycarbonate, polytetrafluoroethylene(TEFLON™), polyvinylchloride (PVC), polydimethylsiloxane (PDMS),polysulfone, and the like. Such polymeric substrates are readilymanufactured using available microfabrication techniques, as describedabove, or from microfabricated masters, using well known moldingtechniques, such as injection molding, embossing or stamping using metalelectroforms, e.g., LIGA methods, or by polymerizing the polymericprecursor material within the mold (See U.S. Pat. No. 5,512,131). Suchpolymeric substrate materials are preferred for their ease ofmanufacture, low cost and disposability, as well as their generalinertness to most extreme reaction conditions. Again, these polymericmaterials may include treated surfaces, e.g., derivatized or coatedsurfaces, to enhance their utility in the microfluidic system, e.g.,provide enhanced fluid direction, e.g., as described in U.S. Pat. No.5,885,470, and which is incorporated herein by reference in its entiretyfor all purposes.

[0022] Substrates can also come in a variety of shapes and forms,including planar forms, e.g., in a chip format, or tubular forms, e.g.,a capillary format. The specific shape will typically vary dependingupon the particular application for which the system is utilized. Forexample, systems employing complex networks of intersecting channels forperformance of multiple successive or parallel integrated operations orreactions typically comprise planar structures to permit theincorporation of the more complex channel networks that are required.Simpler reactions, on the other hand, may be carried out in less complexsystems, e.g., a single channel capillary.

[0023] In particularly preferred aspects, the substrates, and thus theoverall structure of the microfluidic devices used in accordance withthe present invention, are planar. Typically, such devices arefabricated from at least two different planar substrate layers. Thechannel or channels of the device are typically fabricated as groovesinto one surface of one of the substrate layers. A second substratelayer is then overlaid and bonded onto the surface of the first, therebysealing and defining the microscale channels of the device between thetwo layers. Generally, at least one of the substrate layers has one ormore holes or ports disposed through the planar substrate, such that thehole or port is in fluid communication with one or more of themicroscale channels when the substrate layers are mated. These holes orports are typically used both as fluid reservoirs for introducing fluidsinto the channels of the device, as well as providing electrical access,e.g., contact points for electrodes that are placed in electricalcontact with the fluids contained in the device.

[0024] Examples of microfluidic devices employing these planarstructures are described in U.S. Pat. Nos. 5,965,410 and 5,976,336 andU.S. Patent Application No. 60/060,902, filed Oct. 3, 1997, each ofwhich is incorporated herein by reference in its entirety for allpurposes. Three layer substrate structures may also be employed havingan optional third interior layer placed between the first and secondplanar layers, where the interior layer defines the side walls of thechannels of the device while the first and second layers make up the topand bottom walls of the channels, respectively.

[0025] The electrical access ports are useful in heating applications,as is discussed in greater detail below, as well as in the transport anddirection of materials through the channels that are contained in thedevice. In particular, in preferred aspects, the microfluidic devicesand systems that are used in practicing the present invention employelectrokinetic material transport systems. These electrokinetictransport systems utilize controlled electrokinetic forces, e.g.electrophoretic and/or electroosmotic, to controllably move materialsand fluids through the channels and their respective intersections.Examples of controlled electrokinetic transport in microfluidic systemsare described in e.g., published PCT Application No. 96/04547, toRamsey, which is incorporated herein by reference.

[0026] Once the liquid reactants are placed into the microscalechannels, superheating is initiated by applying an effective level of anappropriate energy source for heating the contents of the channel orchannels. A variety of energy sources may optionally be used to heat thefluid within the channels of the microfluidic device. For example, thecontents of the microscale channels may be heated using conductivemethods, e.g., by applying thermal energy to the external surfaces ofthe body structure of the microfluidic element, e.g., substrate orcapillary. A variety of thermal energy sources may be readily utilizedin this capacity. For example, in a simple aspect, the body of thedevice may be placed into an oven or adjacent to or in contact with aheating element, such that the body structure and thus the contents ofthe channels disposed within the body structure are heated tosuperheated levels. Examples of suitable heating elements are well knownto those of skill in the art, and range from simple laboratory hotplates, heating blocks or ovens, to resistive thin film heating elementsthat may be integrated into an internal or external surface of amicrofluidic device or within an appliance adapted for use with thedevice, e.g., into which the device is inserted.

[0027] Alternative energy sources can also be readily utilized inheating the contents of microscale channels, including, e.g., lightsources such as lasers, lamps and the like, which can be directed at thechannels of the device, and preferably, precisely directed at thechannels within a microfluidic device where superheating is desired.

[0028] As noted above, however, in preferred aspects, the microfluidicdevices described herein have electrodes associated with the channels ofthe device. As such, it is generally preferred to utilize electricalenergy in superheating the contents of the channels of the device byresistive methods. Not only does this provide advantages of efficiency,e.g., in using a preexisting energy interface in the electrodes, but italso provides a more precise method of controlling and monitoring thetemperature within the system. Specifically, applying a current throughthe liquid content of a reaction channel results in a resistive heatingof that liquid.

[0029] Electrical resistive heating of fluids in microscale channels isdescribed in substantial detail in U.S. Pat. No. 5,965,410, which isincorporated herein by reference. By applying enough current, e.g., asufficient current density, through a given channel, the contents ofthat channel are superheated. Briefly, electric current passing throughthe fluid in a channel produces heat by dissipating energy through theelectrical resistance of the fluid. Power dissipates as the currentpasses through the fluid, going into the fluid as energy over time toheat the fluid. The following mathematical expression generallydescribes a relationship between power, electrical current, and fluidresistance:

POWER=I²R

[0030] where POWER=power dissipated in fluid; I=electric current passingthrough fluid; and R=electric resistance of fluid. The above equationprovides a relationship between power dissipated (“POWER”), current(“I”) and resistance (“R”).

[0031] Thus, temperature within a given channel can be increased byeither increasing the resistance of the channel or increasing the amountof current passing through the channel, or a combination of the two.Increasing resistance of a channel can be readily accomplished bynarrowing the cross-sectional area of the channel through which thecurrent is applied. Further, by increasing the resistance and/or currentwithin a channel to sufficiently high levels, one can achievesuperheated temperatures within the channels of the device.

[0032] In preferred aspects, sufficient current densities are achievedby using one or both of (1) narrowed channel cross-sectional areas, and(2) increased applied current through the fluid. A simplified example ofa microfluidic device having a channel with a region of narrowedcross-sectional area is shown in FIG. 1. In particular, as shown in FIG.1, a microfluidic device 100 comprises a body structure 102, typicallyfabricated from two overlaid and bonded planar substrates (notseparately shown) where one substrate has a series of channels 104, 106,108 and 110, etched into one planar surface. Overlaying the secondsubstrate provides the cover and sealing wall for the etched channels,forming conduits between the substrate layers. Each of the channelsshown, e.g., channel 106, include a region of narrowed cross-sectionalarea (112) relative to the remaining regions of the channel 114.Reservoirs, e.g., reservoirs 116 and 118, are disposed at the termini ofthe channels, typically as apertures disposed through the overlayingplanar substrate, for fluid introduction and to provide electricalaccess to the channel.

[0033] As noted above, one or both of the channel cross-sectional areaor the applied current can be varied to elevate the temperature of fluidwithin the channel. As such, microscale channels for use in carrying outsuperheated reactions according to the present invention may fall withina wide range of suitable cross-sectional areas. Similarly, the currentsapplied to such channels are similarly widely variable. However, inpreferred microfluidic systems, e.g., those having typical non-heatingchannel dimensions in the microscale range, as set forth above, where itis desired to heat fluids to superheated temperatures, thecross-sectional area of the channels or channel regions in which heatingis desired will typically range from 10 μm² to about 500 μm². Thiscorresponds to channels having dimensions of, e.g., from about 10 μmwide by 1 μm deep, to about 50 μm wide by 10 μm deep. However, wider anddeeper channels my also be used.

[0034] Similarly, currents applied to the fluids within such narrowedchannels typically range from about 5 μA to about 500 μA, and preferablyfrom about 10 μA to about 100 μA.

[0035] The systems of the invention typically include a controlleroperably coupled to the energy source, for monitoring and controllingthe temperature within the reaction channels of the device. This isparticularly useful in those instances where reaction temperatures aredesired that far exceed the expected boiling point of the fluidreactants. Specifically, careful monitoring and control of appliedenergy better allows maintenance of superheated temperatures withoutovershooting the desired temperature and/or inadvertently boiling thefluid reactants, and thereby fouling the channels of the device.

[0036] The controller aspect of the system typically includes aprocessor, e.g., a computer, that is appropriately programmed to receivetemperature data from a sensor placed in thermal communication with thedevice or its fluid contents. The processor is also typically coupled tothe energy source that delivers the heating energy to the device, e.g.,the oven, hot plate, resistive heater, or electrical power supply. Theprocessor is also appropriately programmed to instruct the energy sourceto increase or decrease the amount of applied energy depending uponwhether the sensed temperature of the fluid within the device is aboveor below a set point temperature, e.g., chosen by the user. Theprocessor may further include appropriate programming that indicateswhether the fluid within the device is beginning to boil, e.g., asindicated by a significant, sudden increase in the resistance of thechannel.

[0037] The sensor aspect of the controller is typically coupled to theprocessor, and is in contact with the channels of the device, andpreferably, with the fluid content of those channels. Such sensors mayinclude traditional thermal sensors, such as thermocouples, thermistors,IC temperature sensors. In preferred aspects, however, the temperaturewithin the channels is determined from the conductivity of the fluiddisposed therein, which is dependent in part upon the fluid temperature(See U.S. Pat. No. 5,965,410 and previously incorporated herein). Assuch, the sensor aspect of the controller typically comprises electrodesplaced into electrical contact with different points of the microscalechannels of the device. Preferably, the same electrodes used for heatingand/or for material transport/direction are utilized to determine theconductivity of the fluid, and thus the temperature.

[0038] The methods and systems of the invention have broadapplicability. For example, as noted above, many reactions that progressfaster at higher temperatures can be carried out in accordance with thepresent invention at still faster rates. For example, performance of thepolymerase chain reaction for amplification of nucleic acids generallyutilizes temperatures approaching the boiling point of the aqueousreactants, e.g., in the range of 95 to 100° C., in order to expedite theprocess of denaturing hybridized strands of template nucleic acids.However, such reactions are generally further expedited at superheatedtemperatures, without adverse effects on the overall reaction.

[0039] Similarly, a number of reactions, e.g., enzyme assays, requirethe denaturation of certain enzyme components of the material to betested, prior to performance of the overall reaction, so that thosecomponents do not interfere with the desired reaction. The ability tosuperheat the reaction components, in situ, permits the performance ofsuch denaturation more quickly and efficiently. Similarly, suchsuperheated temperatures are also useful in the destruction and/or lysisof cells for performance of cell-based operations, e.g., preparative oranalytical.

[0040] The present invention is further illustrated with reference tothe following non-limiting examples.

EXAMPLES

[0041] A planar microfluidic device having the channel geometryillustrated in FIG. 1 was used in each of the following superheatingexamples. Reagents were introduced into the channels of the device byplacing the reagents into the reservoirs and allowing capillary actionto draw the reagents through the channels.

[0042] The present invention is further illustrated with reference tothe following non-limiting examples.

Example 1 Conductive Superheating in Microfluidic Systems

[0043] PCR buffer was placed into the channel of the device thatincluded a narrowed region that was 20 μm wide×5 μm deep, by 2 mm long(channel 106 in FIG. 1), and mineral oil was placed over the buffer inthe reservoirs (reservoirs 116 and 118) to reduce evaporative losseswithin the reservoirs.

[0044] Temperature changes within the fluid filled channel weremonitored by measuring the conductivity of the fluid. At roomtemperature, the conductivity measured at 41.8 nA when a 1V potentialwas applied.

[0045] The substrate was placed in an oven at 100° C. for approximatelyone hour, at which point the temperature of the oven was increased above100° C. FIG. 2 shows a plot of the temperature of the fluid within thechannels over the duration of the experiment. The arrows above the plotindicate the point at which the oven temperature was raised to the nextincremental setting. Boiling of the fluid within the reservoirs wasvisually observed at just above 100° C., however, no boiling wasobserved within the channel, as shown by the labeled arrow below theplot. This was confirmed by measuring the conductivity of the fluidwithin the channels after removal from the oven. Specifically,production of bubbles within the channel would have resulted in asubstantial decrease in the conductivity of that channel, as even smallbubbles will significantly constrict the channels used, e.g., havingnarrow dimensions of 20 μm×5 μm. However, conductivity through thechannels did not decrease.

Example 2 Resistive Superheating in Microfluidic Systems

[0046] PCR buffer was again placed into a channel (channel 106) of amicrofluidic device having the channel geometry shown in FIG. 1 asdescribed above, and the conductivity of the buffer at room temperaturewas determined. Electrodes were placed into the reservoirs at thetermini of the channel network. The electrodes were coupled to anelectrical power supply having a 100 μA, 1000V capability, for passingcurrent through the channel network and for concomitantly determiningthe conductivity of the fluid through the channels. The temperature ofthe fluid within the channels was estimated from the conductivity of thesolution using a calibration table.

[0047] The device was placed upon a hot plate at between 65 and 70° C.to elevate the ambient temperature of the device and minimize the amountof current required to superheat the fluid in the channels. The currentapplied through the channel was stepped up over time from a minimum of 2μA to a maximum of 80 μA. During the experiment, the applied current wasstepped up over time to: 2, 5, 10, 20, 30, 40, 50, 55, 60, 65, 70, 75and 80 μA. A plot of fluid temperature (from calibrated conductivity)versus the time period of the experiment is shown in FIG. 3. Thetemperature of the fluid within the channel of the device increased overtime from a measured temperature of 70° C., which was substantiallyequal to the temperature of the hot plate as measured by conventionalmeans, to a temperature of approximately 140 to 150° C. For an aqueousbuffer at or near sea level, this represents superheating of the fluidby 40 to 50° C. The ability to monitor temperature within the channel bythe conductivity through that channel indicates a lack of bubbleformation within the channel, as noted above.

[0048] All publications and patent applications are herein incorporatedby reference to the same extent as if each individual publication orpatent application was specifically and individually indicated to beincorporated by reference. Although the present invention has beendescribed in some detail by way of illustration and example for purposesof clarity and understanding, it will be apparent that certain changesand modifications may be practiced within the scope of the appendedclaims.

What is claimed is:
 1. A system for performing at least one reaction atsuperheated temperature comprising: a microfluidic device comprising: atleast a first substrate; and a microscale channel disposed in the firstsubstrate; a heating system operable to apply energy to the microscalechannel to heat a fluid in the channel to superheated temperatures,without boiling the fluid in the microscale channel; and a controllerwhich is operable to control energy applied from the heating system tothe microscale channel.
 2. The system of claim 1, wherein the firstsubstrate comprises a silica substrate.
 3. The system of claim 2,wherein the silica substrate is a silica capillary.
 4. The system ofclaim 1, further comprising a source of a first reactant.
 5. The systemof claim 4, wherein the source of the first reactant comprises a nucleicacid.
 6. A system for performing at least one reaction at superheatedtemperature, comprising: a microfluidic device comprising: at least afirst substrate having at least a first planar surface, a firstmicroscale channel fabricated into the first planar surface; and asecond planar substrate having at least a first planar surface, thefirst planar surface of the second planar substrate overlaying and beingbonded to the first planar surface of the first planar substrate,thereby defining the first microscale channel therebetween; means forapplying energy to the microscale channel to heat a fluid in the channelto superheated temperatures, without boiling the fluid in the channel;and means for controlling energy applied from the applying energy meansto the microscale channel.
 7. The system of claim 6, wherein at leastone of the first and second planar substrates comprises a silicasubstrate, and the microscale channel is etched into the first planarsurface of the first planar substrate.
 8. The system of claim 6, whereinat least one of the first and second planar substrates comprises apolymeric substrate.
 9. The system of claim 1 or 6, further comprising asensor for determining a temperature of a fluid in the microscalechannel.
 10. The system of claim 9, wherein the sensor comprises aconductivity sensor integrated into the controller.
 11. The system ofclaim 1, wherein the heating system comprises a heating element disposedin thermal contact with the microscale channel for delivering thermalenergy to the microscale channel, the thermal energy heating a fluid inthe channel to a superheated temperature.
 12. The system of claim 6,wherein the means for applying energy comprises a heating elementdisposed in thermal contact with the microscale channel for deliveringthermal energy to the microscale channel, the thermal energy heating afluid in the channel to a superheated temperature.
 13. A system forperforming at least one reaction at superheated temperature, comprising:a microfluidic device comprising: at least a first substrate; and amicroscale channel disposed in the first substrate; means for applyingenergy to the microscale channel to heat a fluid in the channel tosuperheated temperatures, without boiling the fluid in the microscalechannel; and means for controlling energy applied from the applyingenergy means to the microscale channel.